Microwave acoustic delay line

ABSTRACT

A microwave signal is delayed for a predetermined time period with a minimal production of undesired spurious RF signals by providing a resistive termination for undelayed input microwave signals reflected by an acoustic delay means and an output conductive path from the input microwave signals delayed by the acoustic delay means.

United States Paten [1 1 Huang [4 1 Dec. 24, 1974 OTHER PUBLICATIONS Olson-Todays Acoustic (Bulkwave) Delay Lines in The Microwave Journal Vol. 13, No. 3, March 1970; pp. 67-76. 3

P4 22 III? DETECTOR DELAY i S. Tripoli Primary Examiner.,.lames 'W. Lawrence Assistant ExamirierMarvin Nussbaum Attorney, Agent, or Firm-Edward J. Norton; Joseph [5 7 ABSTRACT A microwave signal is delayed for a predetermined time period with a minimal production of undesired spurious RF signals by providing a resistive termination for undelayed input microwave signals reflected by an acoustic delay means and an output conductive path from the input microwave signals delayed by the acoustic delay means. I

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course of or under a contract or subcontract thereunder with the Department of the Air Force.

BACKGROUND OF THE INVENTION 1. Field of the Invention l This invention relates to delay lines and, more particularly to microwave acoustic delay lines.

2. Description of the Prior' Art.

Prior art microwave acoustic delay lines include an acoustic delay device having a transducer capable of converting input microwave energy to acoustic energy and an acoustic medium coupled to the transducer for propagating the acoustic energy in the form of acoustic waves. The prior art acoustic delay device provides a relatively longdelay time per unit length of acoustic medium for 'input microwave signals. However, the prior art transducer also presents a relatively large magnitude impedance mismatch which reflects a substantial portion of the input microwave signal. Thus, a substantial undelayed portion of themicrowave input signal isreflected to the delay line output terminal. For certain applications in the area of microwave communications, particularly electronic countermeasures, the transducer reflected, undelayed microwave signals are most undesirable. A prior art solution to this problem has been to arrange various microwave matching circuitry between the microwave input signal source and'the transducer for reducing the transducer impedance mismatch. This solution has not been entirely successful over relatively wide operating bandwidths since the operating bandwidth of prior art microwave matching circuitry is limited due to the dynamicimpedance characteristic of the transducer. Thus, it is most desirable to suitably arrange microwave components including a microwave acoustic delay device in a microwave delay line which provides a predetermined delay for'microwave signals over a relatively wide frequency bandwidth without reflecting undelayed microwave signals tothe delay line output terminal. v

SUMMARY OF THE INVENTION A predetermined delay for input signals is provided by a delay line having input and output terminals with sampling means coupled to the input terminal for sampling a predetermined portion of the input signal. Detector means coupled to the sampling means detect and convert the sampled input signal to a DC. voltage signal. A first delay means coupled to the detector means delays the D.C. voltage signal for the desired time period. Signal directing means having a first terminal coupled to the sampling means and a second terminal coupled to a second delay means provide a conductive path for the input signal from the sampling means to the second delay means. The second delay means is arranged to delay the input signal for the desired time period. Switching means having terminals coupled to the first delay means, the directing means, and the output terminal is arranged to operate in response to the delayed DC. voltage signal for selectively providing either a resistive termination for undelayed input signals reflected by the-second delay means or a conductive path to the output terminal for the delayed input signal.

BRIEF DESCRIPTION or THE DRAWING FIG. 1 is a schematic of a prior art microwave acoustic delay line having a one port acoustic delay device.

FIG. 2 is a schematic of a microwave acoustic delay line having a one port acoustic delay device arranged according to one embodiment of the invention.

FIG. 3 is a schematic of an alternative switching circuit.

FIG. 4 is a schematic of a microwave acoustic delay line having a one port acoustic delay device arranged according to' another embodiment of the invention.

FIG. 5 is a schematic of a microwave acoustic delay line having a two port acoustic delay device arranged according to another embodiment of the. invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, there is shown a simplified schematic of a prior art microwave delay line 10 using a suitable acoustic device 11' for providing a predetermined delay time for input microwave signals coupled to port 1 of circulator 12. An example of device 11 is an acoustic bulk-wave device having transducer 13 with a surface 16 in contact with acoustic medium 14. Transducer 13 is arranged, as known in the art, to convert microwave orelectromagnetic energy to acoustic energy. The acoustic energy is then propagated in medium 14 in the form of acoustic waves having relatively short acoustic wavelengths which provide relatively long propagation delay per unit length of medium 14. Thus, a'relatively short length of acoustic medium 14 is capable of providing relatively long signal delay. The acoustic waves in medium 14 are reflected back to transducer 13 by a suitable reflective surface 15 or interface at an end of medium 14 opposite transducer surface 16. For example the interface or boundary between air and medium 14 will reflect acoustic waves. The reflected acoustic energy is then converted back to electromagnetic energy by transducer 13. Thus, the total delay provided by device 11 is substantially determined by the time required for the input signal to be convertedto acoustic energy for propagation in the acoustic medium 14 and reflected back to the transducer 13 which converts the acoustic energy back to electromagnetic energy.

Delay line 10 includes a prior art three port circulator 12 arranged to propagate an input signal along a predetermined, non-reciprocal, path from input port 1 to an adjacent port. Circulator 12 is shown in FIG. 1 as directing an input signal in a counterclockwise direction from port 1 to adjacent port 2, and signals present at port 2 are directed in a counterclockwise direction to adjacent output port 3. Thus, under operating conditions, an input microwave signal coupled to port 1 is di rected to port 2 where microwave energy is converted to acoustic energy by transducer 13 terminating port 2. As previously mentioned, the acoustic energy is then propagated in the form of acoustic waves through medium 14 until reflected back to transducer 13 by reflective surface 15. The reflected acoustic waves are converted back to microwave or electromagnetic energy by transducer 13 and the delayed microwave signal is transmitted from port 2 to a load, not shown, terminating output port 3. It will be apparent to those skilled in the art that undelayed microwave signals reflected by Device 11 and delay line is further described in an article entitled .Todays Microwave Acoustic (Bulk Wave) Delay Lines by Frank A. Olson, published in Microwave Journal, Vol. 13, March 1970.

At microwave frequencies, transducer 13 and medium 14 presenta highly reflective load having an impedance with a reactance which must be matched to a signal source impedance for optimum conversion efficiency and minimum signal reflection. It is most undesirable for device 11 to reflect a substantial portion of the undelayed input signal from port 2 to output port 3. Prior art attempts to solve this problem have concentrated' on circuits for improving the impedance mismatch or Voltage Standing Wave Ratio (VSWR) presented by device 11. Such prior art matching circuits have not been entirely successful. Substantial portions of the undelayed input signal are still reflected by device 11 from port 2 'to output port 3.

Referring to FIG. 2, there is shown a schematic of a microwave acoustic delay line according to one embodiment of the invention. Microwave input signal, P is coupled to sampling means for sampling a predetermined portion of the input signal. An example of a sampling means is a prior art four terminal directional coupler 20 arranged to have input-terminal 21, first output 28. The reflected, and undelayed portion of the input signal, P,, is fed in a counterclockwise direction along a non-reciprocal path from port 2 of circulator 28 to switching circuit 27 coupled to port 30f circulator 28.

Switching circuit 27 includes a suitable single pole single throw switch 31 arranged to be in a normally closed position in the absence of DC. bias signal V Thus, in the absence of DC. bias signal, V switch 31 provides a conductive path to resistive or signal absorbing load 32 for undelayed signal P... As previously mentioned, the DC. bias signal V for switching circuit 27 is delayed by video delay line 26 for a time period substantially equal to the delay provided by device 30 and switch 31 is in a normally closed position permitting the undelayed reflected signal P,- and any other undesired microwave signal leaked from port 1 to port 3 of circulator 28 to be absorbed by load 32.

At the end of the delay time provided by device 30, a delayed microwave signal, P is fed from port 2 to switching circuit 27 terminating port 3 of circulator 28.

terminal 22, second output terminal 23 and isolated or resistive terminated terminal 24. Coupler is' arranged to have a coupling coefficient which permits a relatively small portion of input signal P or signal P to be fed to detector means connected to terminal 22 and the remainder'of input signal P to be fed to four port circulator 28. Detector 25 is a prior art device such as a crystal or diode detector arranged to detect and convert microwave signals to a corresponding DC. voltage signal, V. The DC voltage signal, V, from detector 25 is coupled to a suitable video delay line 26 arranged-to delay the DC. voltage signal, V, for a predetermined time. The delayed DC. voltage signal, V is a bias signal suitable for activating or operating switching circuit 27. Thus, it is understood that the coupling Coefficient of coupler 20 and the sensitivity of detector 25 are'selected to provide a DC. voltage having a polarity and magnitude suitable for. operating switching .circuit 27. The function of switching circuit 27 will be described below.

The signal P',,. is the portion of input signal P which is fed from coupler terminal 23 to port 1 of circulator 28. Circulator 28 is a signal directing means arranged to provide a first non-reciprocal conductive path from circulator port 1 to delay device terminating circulator port 2 and a second non-reciprocal path from delay device 30 to switching circuit 27. The function of circulator.28 is as follows. Signal P;,, is fed in a counterclockwise direction along a non-reciprocal path from port 1 to port 2 of circulator 28. The transducer 29 of a prior art acoustic delay device 30, similar to device 11 in FlG. l, is coupled to port 2 of circulator 28. Delay device 30 is arranged to provide substantially the same delay for microwave signals as delay circuit 26 provides for DC. signalV. Transducer 29 includes a prior art matching circuit, not shown, arranged to transform the impedance at port 2 of circulator 28 to the impedance of device 30 at the frequency, f,,, of the input signal P,,,. Part of the input signal coupled to device 30 is converted to acoustic energy by transducer 29 and part of the input signal is reflected back to port 2 of circulator It should be noted that since the delay provided by delay circuit 26 is substantially equal to the delay provided by delay device 30, the switching circuit bias signal, V and the delayed microwave signal, P are simultaneously coupled to switch 31. The bias signal, V

causes switch 31 to open and present an open circuit or a relatively large magnitude reflective impedance to delayed microwave signal, P,,,. The signal, P reflected by switching circuit 27 is fed in a counterclockwise direction along a non-reciprocal path from port 3 to the output port or port 4 of circulator 28.

Referring to FIG, 3, there is shown a schematic of a switching circuit 40 arranged as an alternative embodiment to switching circuit 27 in FIG. 2. Switch 41 is a single pole single throw switch having an input terminal 43 connected both to resistive termination or signal absorber 42 and port 3 of circulator 28 in FIG. 2 and an output terminal 44 connected to ground potential. Under operating conditions, switch 41 is in a normally open position in the absence of applied bias signal, V from delay circuit 26. Thus, the signal, P,, is fed along a conductive path to signal absorbing load 42. As described in FIG. 2, the delayed microwave signal, P

from device 30 is fed from port 2 to switching circuit 40 terminating port 3 of circulator 28. The delayed bias signal, V and the delayed microwave signal, P are simultaneously coupled toswitch 41. The applied bias signal V causes switch 41 to close and present a relatively low impedance path to ground potential or a reflective mismatch at port 3 of circulator 28. The delayed signal, P is reflected by the reflective mismatch presented by switching circuit 40 at port 3 and is fed as signal P from port 3 to a load, not shown, terminating output port 4 of circulator 28.

Referring to FIG. 4, there is shown a schematic of another embodiment of a microwave acoustic delay line according to the invention. Microwave acoustic delay line 50 has first and second substantially similar bulk wave acoustic delay devices 51 and 52 arranged to provide substantially the same delay for applied microof coupler 20 in FIG. 2 so that a relatively small portion of input signal, P,,,, or signal P is fed to prior art detector 58 (described in FIG. 2) connected to terminal 56. The signal P',,, is the portion of input signal P, which is fed to input terminal 60 of prior art directional coupler 59. As described in FIG. 2, detector 58 converts microwave signals to a corresponding-D.C. voltage signal, V, which is in turn coupled to a suitable video delay line 64. Video delay line 64 is arranged, as known in the art, to delay voltage signal V for a predetermined time. The delay provided by delay line 64 is substantially equal to the delay provided by devices 51 and 52. The polarity and magnitude of delayed voltage signal, V from delay line 64 is suitable for activating or operating switching circuit 60. The function of switching circuit 60 will be described below.

Directional coupler 59 is arranged to have input terminal 60, first output terminal 62, second output terminal 61 and isolated terminal 63. Unlike couplers 53 and 20, coupler 59 is formed to provide a transmission path for substantially equal portions of signal P',,, from input terminal 60 to first and second output terminals 61 and 62 respectively. Thus,.signal P at terminal 62 is substantially equal in magnitude to signal P, at terminal 61. Also, the relative phase difference between the signal I at terminal 62 and the signal P, at terminal 61 is sub stantially 90.

Itis well known in the art that directional coupler 59 described above, will sum two input signals (P and P or P, and P simultaneously coupled to output terminals 61 and 62 provided such signals have a relative phase difference of substantially 90. The sum of signals P and P or P and P,,is fed to terminal 63 as signal P,. P 1 is the delayed portion of signal P reflected back to terminal 62 by device 51 and signal P is the delayed portion of signal P, reflected back to terminal 61 by device 52. Similarly, signal P is the undelayed portion of signal P; reflected back to terminal 62 by device 51 and signal P is theundelayed portion of signal P, reflected back to terminal 61 by device 52. Coupler 59 and devices 51 and 52 are arranged, as known in the art, so that the relative phase difference between signals P, and P and the relative phase difference between P I and P,,, 2 is substantially 90. Therefore, as described above, the sum of P, and P and the sum of P 1 and P is fed to coupler terminal 63.

Input terminal 65 of switching circuit 60 is coupled to coupler terminal 63. Switching circuit 60 includes a single pole double throw switch 66 having a first output terminal 67 coupled to a resistive termination 68 and a second output terminal 69 coupled to a delay line terminating load, not shown. Switching circuit 60 operates to provide a selectable conductive path between tenninal 65 and either terminal 67 or 69 in response to delayed D.C. voltage signal, V from video delay line 64.

Under operating conditions, switch 66 provides a conductive path for the sum of undelayed reflected signals P', 1 and P,., from terminal to resistive terminaand P ,are simultaneously coupled to switching circuit 60 causing switch 66 to provide a conductive path for the sum of delayed signals P 1 and P 2 from terminal 65 to output terminal 69.

Referring to FIG. 5, there is shown a microwave acoustic delay line 70 having a prior art two port microwave acoustic delay device 71 for providing a predetermined delay time for input microwave signals, P Delay line 70 includes a sampling means or directional coupler 72 having a first output terminal '73 coupled to detector 74 and a second output terminal 75 coupled to input port 76 of delay device 71. Directional coupler 72 is arranged to operate similar to directional couplers 20 and 53 described in FIGS. 2 and 4. Thus, coupler72 directs a predetermined portion, P of the input signal, P to detector 74 which in turn detects and converts signal P; to a D.C. voltage signal V. The remainder of the input signal P,,,, represented as signal P,,,, is coupled to input port 76 of delay device 71.

The voltage signal V from detector 74 is coupled to video delay line 77 arranged to delay the D.C. voltage V signal for a predetermined time. The delayed D.C. voltage signal, V is a bias signal suitable for activating or operating switching circuit 78. Thus, similar to FIGS. 2 and 4, it is understood that the coupling coefficient of coupler 72 and the'sensitivity of detector 74 are selected to provide a D.C. voltage having a polarity and magnitude suitable for operating switching circuit 78 coupled to port 2 of circulator 81. The function of switching circuit 78 will be described below.

The signal P' is delayed by the microwave acoustic delay device 71 for a period equal to the delay provided by video delay line 77. The delayed output signal, P from delay device 71 is coupled to port 1 of a suitable circulator 81 arranged to provide a non-reciprocal conductive path for signal P from port 1 to switching circuit 28 coupled to circulator port 2. Delay device 71 and circulator 81 are arranged so that the delayed output signal P from delay device 71 and the delayed D.C. bias signal, V from video delay line 77 are simultaneously coupled to switching circuit 78. Under operating conditions, switching circuit 78 selectively provides either a conductive path to signal absorber or a reflective mismatch. Switching circuit 78 includes a single pole, single throw switch 79 which, in the absence of a D.C. voltage signal, is in a normally closed position. Therefore, since the D.C. bias signal, V is de layed and not yet coupled to switch 79, a conductive path is provided for undelayed portions of signal P',,, (referred to as P to load or signal absorber 80 terminating switch 79. Switch 79 is in a normally open position in response to an applied D.C. bias signal. Thus, an open circuit or reflective mismatch is presented by switching circuit 78 in response to the applied D.C. bias voltage signal V The reflective mismatch terminating port 2 of circulator 81 then reflects the delayed signal P, from port 2 to a load, not shown, terminating circulator port 3.

In summary, a microwave acoustic delay line is described in FIGS. 2, 4 and 5 which provides a resistive termination 32 and 68 or absorption of undelayed microwave signals reflected by an acoustic delay device 30, 51 and 52 and a conductive path to an output port for microwave signals delayed for a predetermined time, T by the microwave acoustic delay device 30, 51 and 52. The microwave acoustic delay line is arranged to provide means 20 and 53 for sampling a predetermined portion of the input microwave signal and means 25 and 58 for detecting the sampled input microwave signal to a DC voltage signal, V. The DC. voltage signal, V, is first delayed by a video delay line 26 and 64 for the predetermined delay time, T and then coupled to a switching circuit 27 and 60. In the absence of DC. voltage signal, V,,, the switching circuit 27 and 60 is arrangedto provide a conductive path to resistive termination 32 and 68 or microwave signal absorber for the undelayed signals, P,, P, 1 and P, 2 reflected by the acoustic delay device 30, 51 and 52. Upon the simultaneous application of delayed DC bias voltage V and delayed microwave signal P P 1 and P the switching circuit 27 and 60 provides a conductive path for the delayed microwave signal to a delay line output terminal.

I The microwave acoustic delay line according to the invention is preferably operated in response to a pulsed input microwave signal. However, the acoustic delay line may operate in response to a C.W. or continuous wave input microwave signal providing the duration of the C.W. signal does not exceed the delay provided by the acoustic device 30, 51 and 52.

Embodiments of the invention have been shown and described in FIGS. 2 and 4 only by way of example. Various other embodiments and modifications thereof will be apparent to those skilled in the art, and will fall within the scope of the invention as defined in the following claims.

' What is claimed is:

l. A delay line having an input terminal and an output terminal for providing a predetermined delay for input signals, comprising:

sampling means coupled to said input terminal for sampling a predetermined portion of said input signal;

detector means coupled to said sampling means for detecting and converting said sampled input signal to' a DC. voltage signal;

first delay means coupled to said detector means for delaying said DC. voltage signal, for said predetermined delay;

second delay means providing a delayed portion and an undelayed portion of said input signal;

signal directing means having a first terminal coupled to said sampling means and a second terminal coupled to Said second delay means, said directing means providing a conductive path for said input signal from said sampling means to said second delay means; and

switching means coupled to said first delay means, a

third terminal of said directing means and said output terminal, said switching means operating in response to said delayed DC. voltage signal to selectively provide a signal absorbing termination for said undelayed portion of said input signal and a conductive path to said output terminal for said delayed portion of said input signal.

2. A delay line according to claim 1, wherein said sampling means is a directional coupler.

3. A delay line according to claim 1, wherein said detector means is a diode detector.

4. A delay line according to claim 1, wherein said first delay means is a video delay line.

5. A delay line according to claim 1, wherein said second delay means is an acoustic delay means.

6. A delay line according to claim 5, wherein said acoustic delay means is a one port microwave acoustic bulk delay line.

7. A delay line according to claim 1, wherein said switching means is a single pole double throw switch operating in response to said delayed DC. voltage signal and having an input terminal coupled to said third terminal of said directing means and first and second selectable output terminals, said switch having said second output terminal connected to said signal absorbing termination and said first output terminal connected to a reflective termination for said delayed portion of said input signal whereby, said switch provides a conductive path for said delayed portion of said input signals from said switch input terminal to said reflective termination in the presence of said delayed DC. voltage signal and a conductive path for said undelayed portion of said input signals from said switch input terminal to said signal absorbing load at all othertimes.

8 A delay line according to claim 1, wherein said signal directive means is a four port circulator means directing said input and said delayed portion of said input signals along predetermined non-reciprocal paths, said circulator means having a first terminal coupled to said sampling means, a second terminal coupled to said second delay means, a third terminal coupled to said switching means and a fourth terminal for coupling out said delayed portion of said input signal.

9. A delay line having an input terminal and an output terminal for providing a predetermined delay for input signals transmitted between said input andoutput terminals, comprising:

sampling means coupled to said input terminal for sampling a predetermined portion of said input signal; I

detector means coupled to said sampling means for detecting and converting said sampled input signal to a DC. voltage signal;

first delayv means coupled to said detector meansfor providing said predetermined delay for said DC. voltage signal; second delay means for delaying said input signal for said predetermined delay;

directive means having first, second, third and fourth terminals, said first terminal being coupled to said sampling means and said second terminal being coupled to said second delay means, said directive means providing a directive non-reciprocal path for said input signal to said second terminal of said directive means; and

switching means coupled to said first delay means and said third terminal of said directive means, said switching means operating in response to said delayed DC. voltage signal to selectively provide a signal absorbing termination for undelayed input signals reflected by said second delay means and a conductive path from said third terminal to said fourth terminal of said directive means for said delayed input signal.

10. In combination,

an acoustic delay means which provides in response to an input signal applied thereto a first signal which is said input signal delayed a predetermined time period and a second signal which is a portion of said input signal undelayed by-said delay means,

said sampling and delaying means to provide in response to said control signal and a signal absorbing termination for said second signal and an output path from said delay means to a utilization means for said first signal. 

1. A delay line having an input terminal and an output terminal for providing a predetermined delay for input signals, comprising: sampling means coupled to said input terminal for sampling a predetermined portion of said input signal; detector means coupled to said sampling means for detecting and converting said sampled input signal to a D.C. voltage signal; first delay means coupled to said detector means for delaying said D.C. voltage signal, for said predetermined delay; second delay means providing a delayed portion and an undelayed portion of said input signal; signal directing means having a first terminal coupled to said sampling means and a second terminal coupled to said second delay means, said directing means providing a conductive path for said input signal from said sampling means to said second delay means; and switching means coupled to said first delay means, a third terminal of said directing means and said output terminal, said switching means operating in response to said delayed D.C. voltage signal to selectively provide a signal absorbing termination for said undelayed portion of said input signal and a conductive path to said output terminal for said delayed portion of said input signal.
 2. A delay line according to claim 1, wherein said sampling means is a directional coupler.
 3. A delay line according to claim 1, wherein said detector means is a diode detector.
 4. A delay line according to claim 1, wherein said first delay means is a video delay line.
 5. A delay line according to claim 1, wherein said second delay means is an acoustic delay means.
 6. A delay line according to claim 5, wherein said acoustic delay means is a one port microwave acoustic bulk delay line.
 7. A delay line according to claim 1, wherein said switching means is a single pole double throw switch operating in response to said delayed D.C. voltage signal and having an input terminal coupled to said third terminal of said directing means and first and second selectable output terminals, said switch having said second output terminal connected to said signal absorbing termination and said first output terminal connected to a reflective termination for said delayed portion of said input signal whereby, said switch provides a conductive path for said delayed portion of said input signals from said switch input terminal to said reflective termination in the presence of said delayed D.C. voltage signal and a conductive path for said undelayed portion of said input signals from said switch input terminal to said signal absorbing load at all other times.
 8. A delay line according to claim 1, wherein said signal directive means is a four port circulator means directing said input and said delayed portion of said input signals along predetermined non-reciprocal paths, said circulator means having a first terminal coupled to said sampling means, a second terminal coupled to said second delay means, a third terminal coupled to said switching means and a fourth terminal for coupling out said delayed portion of said input signal.
 9. A delay line having an input terminal and an output terminal for providing a predetermined delay for input signals transmitted between said input and output terminals, comprising: sampling means coupled to said input terminal for sampling a predetermined portion of said input signal; detector means coupled to said sampling means for detecting and converting said sampled input signal to a D.C. voltage signal; first delay means coupled to said deteCtor means for providing said predetermined delay for said D.C. voltage signal; second delay means for delaying said input signal for said predetermined delay; directive means having first, second, third and fourth terminals, said first terminal being coupled to said sampling means and said second terminal being coupled to said second delay means, said directive means providing a directive non-reciprocal path for said input signal to said second terminal of said directive means; and switching means coupled to said first delay means and said third terminal of said directive means, said switching means operating in response to said delayed D.C. voltage signal to selectively provide a signal absorbing termination for undelayed input signals reflected by said second delay means and a conductive path from said third terminal to said fourth terminal of said directive means for said delayed input signal.
 10. In combination, an acoustic delay means which provides in response to an input signal applied thereto a first signal which is said input signal delayed a predetermined time period and a second signal which is a portion of said input signal undelayed by said delay means, means for applying said input signal to said delay means, means for sampling said input signal to provide a control signal and for delaying said control signal said predetermined time period, and means coupled to said acoustic delay means and to said sampling and delaying means to provide in response to said control signal and a signal absorbing termination for said second signal and an output path from said delay means to a utilization means for said first signal. 